Beginning Friday, an AT&T customer was impacted by a denial-of-service attack stemming from IP addresses connected to img.4chan.org. To prevent this attack from disrupting service for the impacted AT&T customer, and to prevent the attack from spreading to impact our other customers, AT&T temporarily blocked access to the IP addresses in question for our customers.

This action was in no way related to the content at img.4chan.org; our focus was on protecting our customers from malicious traffic. Overnight Sunday, after we determined the denial-of-service threat no longer existed, AT&T removed the block on the IP addresses in question. We will continue to monitor for denial-of-service activity and any malicious traffic to protect our customers.

According to this discussion - thanks to a commenter on the original story - other ISP engineers took similar action because of a distributed denial of service attack. The engineer in this case claims he took the action in agreement with the owner of 4chan's hosting service.

Denial of service attacks take several forms but are a way of overwhelming a webserver, usually, with massive numbers of requests so that it slows or goes offline completely. Attackers can also use automated reply responses to go after other sites by faking the source address information. Update:

He said as a result of action taken to fight off a DDOS attack some AT&T customers were getting errant traffic from a 4chan switch.

Moot wrote: "In the end, this wasn't a sinister act of censorship, but rather a bit of a mistake and a poorly executed, disproportionate response on AT&T's part. Whoever pulled the trigger on blackholing the site probably didn't anticipate [nor intend] the consequences of doing so.

We're glad to see this short-lived debacle has prompted renewed interest and debate over net neutrality and internet censorship."

Finally he added: "PS: If any companies would like to hook us up with some better hardware, feel free! The architecture we've got powering this large and influential beast is really quite embarrassing. ( ._.)"